Incessant excitation of the Earth's free oscillations: global comparison of superconducting gravimeter records

Records of superconducting gravimeters (SGs) at Canberra (Australia), Esash i (Japan), Metsahovi (Finland) and Syowa Stations (Antarctica) were analyze d to search For further evidence of background free oscillations of the Ear th. Spectrograms for 1-year period and averaged power spectra for seismical ly quiet periods were obtained fur each of the stations. Anomalous features of the oscillations observed at Syowa Station, such as an apparent seasona l variation and a high intensity at frequencies between 3 and 4 mHz, were a bsent at the other SG stations. Among the SG stations used in this study, t he background free oscillations were detected most consistently and distinc tly at Canberra, where the noise level was comparable to that at the IDA qu ietest station, while that at Syowa Station was close to the critical limit for detecting the oscillations. The background free oscillations provide a good reference to evaluate the noise level in the milliHertz band. (C) 200 0 Elsevier Science B.V. All rights reserved.